Microsoft has confirmed the official name of their Windows Phones, Microsoft Lumia will be the brand that Microsoft will use to replace the Nokia Lumia brand.

Microsoft Lumia is the new brand name that takes the place of Nokia for the software maker. The name change follows a slow transition from Nokia.com over to Microsoft's new mobile site, and Nokia France will be the first of many countries that adopt "Microsoft Lumia" for its Facebook, Twitter, and other social media accounts. Microsoft has confirmed to The Verge that other countries will follow the rebranding steps in the coming weeks.
Source The Verge